I'm trying to better understand the role of a service host and coming from a perl/ruby background the java and .NET examples are a little unfamiliar. Can anyone shed some light on where a service host fits into a typical Rack/RubyOnRails setup? It seems like this encompasses more than just a Rack compatible webserver. Would Passenger be a good example of a service host or some kind of an nginx+unicorn configuration? Or does a web framework like Rails or Sinatra itself fill some of these responsibilities.